Firstly this invention relates to a method for using a revolver crane for repositioning a load, said revolver crane comprising a stationary support ring assembly, a movable platform ring for rotating the crane placed hereon, a boom comprising hoisting elements, being slewable for changing its outreach and which is bearing-mounted on the platform ring, a mast which is also bearing-mounted on the platform ring and further comprising a ballast, which is connected to the mast and which is positioned on the platform ring.
Revolver cranes are commonly used for performing heavy hoisting activities. By means of the support ring assembly the crane is supported by a large ground area, as a consequence of which heavy loads can be hoisted. By transpositioning of the platform ring around the support ring assembly, combined with the variable outreach of the boom, the revolver crane is issued with a large circular working area.
Revolver cranes are among other things used for building, renovation and dismantling of petrochemical and chemical plants as well as for building power stations. In order to be able to absorb the load""s moment, which arises from these activities, known revolver cranes will comprise heavy ballast (of the order of 1500 tons). This ballast has a permanent position on the platform ring and serves to yield a counter moment which balances the load moment originating from the load. Because of the heavy ballast the costs of transport of such revolver cranes are excessive.
When transpositioning a load by means of such a revolver crane it is proceeded such that together with the increasing of the load moment (by raising the load and increasing the outreach of the boom) the force which is exerted by the ballast on the mast, and therefor the counter moment also, increases. Since the ballast has a fixed position on the platform ring, the mast of the ballast must be sufficiently large for being able to exert a sufficient counter moment in all cases. This is the reason why such known revolver cranes have an extremely high ballast weight.
The invention aims at a method for using a revolver crane for transporting a load with which a revolver crane can be used in a more effective way.
To this end the method according to the invention is characterized by the subsequent steps of:
a. positioning the support ring assembly in relation to the load to be transported, such that the hoisting elements can be connected to the load, while the boom is in a position of small outreach;
b. raising the load by means of the hoisting elements;
c. slewing the boom so as to increase its outreach, while the ballast is at the same time raised from the platform ring and, by slewing the mast, moving it into a position outside the platform ring such that the moment exerted by the load and the moment exerted by the ballast are in balance;
d. putting down the load on the intended location, after which the hoisting elements are kept connected to the load;
e. transporting the ballast, by slewing the mast, into a position inside in relation to the platform ring and repositioning the ballast on the platform ring;
f. finally disconnecting the hoisting elements from the load and, if necessary, slewing the boom so as to decrease its outreach.
Unlike the known method, wherein the ballast has a fixed position on the platform ring and keeps this fixed position, with the method according to the invention it is such acted as to change the position of the ballast in relation to the platform ring. An important result is that the amount of ballast can be decreased importantly in relation to the known revolver cranes. In fact, in similar hoisting operations, a ballast of about 900 tons can be used instead of a conventional ballast of 1500 tons.
After putting down the load the ballast must be repositioned to its original position on the platform ring. Should the load be disconnected from the hoisting elements immediately after having been put on the intended position, it would no longer be possible to place the ballast back on the platform ring by only using the crane itself. It then would be necessary to transport the ballast in pieces back to the platform ring, which would be very labour intensive. According to the invention the load, after having been put down on the intended position, is being kept connected to the hoisting elements, such that the load produces the counter moment necessary for repositioning the ballast on the platform ring. The more the ballast is repositioned to the inside, the more the counter moment which is exerted by the load (i.e. the force exerted by the hoisting elements) decreases since the load will rest on its support more and more. Finally the ballast reaches its original position on the platform ring after which the connection between the hoisting elements and the load can be ended and, if necessary, the boom can be slewed back to its original position having a small outreach.
Although the main claim relates to a method wherein a load is repositioned outwardly in relation to the revolver crane, it is also possible by reversal of the subsequent steps to reposition a load inwardly in relation to the revolver crane. The essence of the invention, to wit repositioning the ballast outwardly, resp. inwardly, so as to counter balance the moment exerted by a load, remains unchanged.
According to a preferred embodiment of the invention during step c. the ballast is released to a position shortly above the ground and kept in this position. In essence this is a safety measure so as to prevent that in an emergency situation the ballast falls down over too large a distance, for example when, before the load has been put down on the intended location, the connection between the load and the hoisting elements unintentionally is interrupted. If in such a case the ballast is on too high a distance above the ground the crane might fall over.
Of course there is a possibility that during step c. a rotation of the platform ring in relation to the support ring assembly is carried out.
The invention also relates to a revolver crane for performing the method according to the invention, comprising a stationary support ring assembly, a platform ring which is drivable on said assembly for rotating the crane, a boom comprising hoisting elements which is bearing-mounted on the platform ring and which can be slewed for changing its outreach, a mast which is bearing-mounted on the platform ring as well, as well as a ballast which is connected with the mast and which is positioned on the platform ring.
The revolver crane according to the invention is characterized in that the ballast is connected to the mast by means of ballast hoisting elements and which can be raised from the platform ring with said ballast hoisting elements, which mast is slewable for changing its outreach and in that load and moment monitoring means are used for monitoring load and counter momenta exerted by the boom, the load connected to the boom as well as by the ballast, respectively and that control means are used for controlling the outreach of the boom and the mast, respectively, depending on the moments.
With aid of the revolver crane according to the invention of which the ballast does not, contrary to the state of the art, have a fixed position on the platform ring, the method according to the invention can be performed easily. The load and moment monitoring means monitor the load moment which is exerted on the boom and the counter moment which is exerted on the mast and take care for balancing these moments. To this end control means which depend on the load and moment monitoring means are provided.
It is common with the revolver crane of the present type that the mast is connected to the platform ring by means of a pull-in hoist. In such a case it is preferred that the load and moment monitoring means comprise part of the pull-in hoist. In case the load on the pull-in hoist is becoming too high this indicates that the load moment on the boom is too high; otherwise, in case the load on the pull-in hoist is too low it means that the counter moment on the mast is too high. In both cases a compensation has to be given for repositioning the ballast.
Furthermore it is mentioned that the control means may be positioned decentrally with regard to the respective driving mechanisms. By drive mechanisms are meamt among other things those mechanisms that take care of the slewing of the boom and the mast, respectively as well as for activating the ballast hoisting means.
Such a control usually is arranged centrally by a large amount of wires that lead from a central position to the driving means. The decentralized control as presented here has a number of advantages. On the one hand the construction becomes better organized, more reliable as well as less prone to breakdown. Besides these advantages there is less chance of leakage and repair and maintenance can be carried out more easily.
Finally there is a possibility that the revolver crane is constructed from segments which can be transported by standard containers. In this way the revolver crane complies with container specification when transported, which means that use can be made of containers. In this way transport is cheaper and faster. Furthermore less storage room is necessary, less people are needed during transport and use can be made of well organized logistics of container organisations.